Can anyone explain what specifically makes it alpha quality? What is the danger in me using a custom build from that tool? Or is it just that it hasn't been used thoroughly enough to know what some dangers might be?

I'm really trying to reduce the size of my site, and this builder would be a big help since there are several pieces I can remove. I'd be happy to help test the build if needed, but there's just no clear indication on why it is alpha quality (it's also frustrating that it's listed so prominently on the download page, and when you get to the actual builder it recommends not using it in production).